This invention relates to automotive tire parameter monitoring systems. More particularly, this invention relates to a tire parameter monitoring system powered by inductively generated D.C. electrical power.
Tire parameter monitoring systems are known and are commonly used to monitor one or more parameters of interest in individual pneumatic tires of a vehicle and to provide an advisory signal to the driver, or an on-board computer system, whenever the monitored parameter in one or more of the vehicle tires reaches a value lying outside of a predetermined safe range. The parameter is typically internal tire pressure, tire temperature, internal tire air temperature, lateral tire force, or some other parameter of interest. The advisory signal is typically generated by an r.f. signal generator controlled by a microprocessor connected to the tire parameter sensor, the advisory signal being generated whenever the tire parameter measured by the sensor lies outside a predetermined normal operating range, signifying either a high or a low parameter condition. This r.f. signal is transmitted to a vehicle-mounted receiver, which uses the advisory signal to alert the driver either visually (by activating a warning lamp or display) or audibly (by activating an audible alarm) or both. Electrical power to the sensor circuitry is usually provided by a battery, which must be replaced (if possible) when the available battery power drops below a useful level. In some known systems, the battery cannot be replaced so that the entire sensor assembly must be replaced when the battery has reached the end of its useful lifetime.
A tire parameter sensor system which monitors internal tire pressure is disclosed in commonly assigned, co-pending patent application Ser. No. 10/346,490 filed Jan. 21, 2003 for “External Mount Tire Pressure Sensor System” (the '490 application), the disclosure of which is hereby incorporated by reference. This system uses a mechanical strain sensor having an essentially linear variable resistance characteristic in one branch of an electrical bridge circuit to measure the internal pressure of a tire to which the sensor is attached. This type of sensor is relatively insensitive to mechanical vibrations, which are regularly encountered in an automotive environment. In addition, the configuration of the electrical circuitry (i.e., the electrical bridge circuit) is relatively simple, has well-known performance characteristics, and has been found to be reasonably reliable in operation. However, the useful lifetime of this sensor system is limited by the energy capacity of the battery used to electrically power the system. More particularly, since the sensor circuitry is continuously powered by the essential battery when the vehicle electrical system is activated, the useful lifetime of the battery is limited by the battery energy capacity and the total activation time. This drawback is compounded by the need for components having relatively small physical size due to installation constraints. As a consequence, the size of the battery is severely limited, and battery replacement is a major constraint to the installed efficacy of such known sensor systems.
Commonly assigned, co-pending patent application Ser. No. 11/267,775 filed Nov. 4, 2005 for “Tire Pressure Sensor System With Improved Sensitivity And Power Saving”, the disclosure of which is hereby incorporated by reference, discloses a tire pressure monitoring system which incorporates a power saving unit providing extended useful battery life. The power saving unit limits the application of electrical power from a battery to the system in a manner related to tire speed so that power is only applied, and thus drawn from the battery, for a measurement period related to tire speed after the tire speed has reached a threshold speed value. Preferably, this measurement period is related to the time required for a tire of a given size to complete a preselected number of revolutions. Since battery power is not continuously applied to the system, the useful lifetime of the battery is extended beyond that of the same type of battery used in a tire parameter monitoring system in which the battery is continuously electrically connected to the system components.
While the power saving unit described above does provide a substantial improvement in the useful lifetime of the system battery, and thus the overall system, all batteries have a finite energy capacity and eventually will be depleted. Once depleted, the battery must be replaced, if possible. If the battery cannot be replaced due to the design of the system, a new system must be installed.
Battery powered tire parameter monitoring systems also suffer from further disadvantages. First, the cost of the battery relative to the other system components is relatively high. The cost of a high quality lithium battery, for example, is approximately one-half the cost of the entire sensor assembly. In addition, the battery adds appreciable weight to the system: essentially fifty percent of the system weight is due to the battery.
Efforts have been made to design fire parameter sensor systems which eliminate the need for a battery to supply electrical power to the system components which require such power. One such design is shown in U.S. Patent Application Publication No. US 2004/0244474 A1 published Dec. 9, 2004, the disclosure of which is hereby incorporated by reference. This design utilizes a specially designed hub assembly at each wheel, the hub assembly having a stator element and a rotor element with permanent magnets and coils for electromagnetically generating an A.C. supply voltage for the sensor elements located on or in the wheel. While effective in eliminating the battery, this design requires that each wheel be provided with the special hub assembly, which adds substantial expense to the sensor system and requires substantial technical expertise to install on the vehicle.
Another approach to designing a tire parameter sensor system devoid of a battery is illustrated in FIG. 1, which shows a known system 10 using RFID tags configured as passive transponders. As seen in this FIG., each tire of a vehicle has an associated RFID tag attached thereto. Thus, left front (L.F.) tire 11 has a tag 12; right front (R.F.) tire 13 has a tag 14; left rear (L.R.) fire 15 has a tag 16; and right rear (R.R.) tire 17 has a tag 18. Each tag 12, 14, 16, 18 has a small antenna (typically a dipole antenna-not shown) for enabling the electro-magnetic transfer of power into the tag. Positioned closely adjacent each tag 12, 14, 16, 18 is an associated antenna 20, 21, 22, 23. Each antenna 20, 21, 22, 23 is connected to a central transceiver 25, which controls system operation. Each tag 12, 14, 16, 18 is configured as a passive transponder, which derives its electrical operating power from electromagnetic energy received from the associated antenna when that antenna is activated by the central transceiver 25. While this system design eliminates the need for a separate battery for each tag 12, 14, 16, 18, it suffers from two major disadvantages. First, each antenna must be positioned closely adjacent the corresponding tag due to the relatively short effective energy transfer range inherent in an RFID tag system. This requirement imposes a severe constraint on the antenna locations, which must be precisely positioned with respect to the dipole antenna carried by the tags. Second, electrical cabling must be routed between the central transceiver 25 and the individual antennae 20, 21, 22, 23. This imposes a requirement of careful routing of the cables to avoid mechanical abrasion, electrical interference, and thermal stresses over time.
Efforts to provide a battery-less tire parameter sensor system devoid of the above-noted disadvantages have not been successful to date.